Feels rot me like the 'Heal from 0' rules work as the designer intended. They would have put penalty on that if they really wanted to make it less optimal, or make bonus action healing better so healing before the ally drops to 0 would be a better move.
But, if you think its too easy, you could always just deny the healed character an action on its first turn. They're not defenceless or anything (staying down would be just as dangerous at low HP than at 0 because of the advantage) and they could get up, but they would still be shaken enough not to be able to just smack a dude right away. I think adding more complications would lead to more bookkeeping than is necessary and just cause the Heal Bot play style to come back, with paranoid players spending their turns making sure no one drop instead of contributing to ending the fight faster.